Since the early 1960s, researchers have been experimenting with creating directive low-frequency sound from nonlinear interaction of an aimed beam of ultrasound waves produced by a parametric array using heterodyning. Ultrasound has much shorter wavelengths than audible sound, so that it propagates in a much narrower beam than any normal loudspeaker system using audio frequencies.

The first modern device was created in 1998, and is now known by the trademark name “Audio Spotlight”, a term first coined in 1983 by the Japanese researchers who abandoned the technology as infeasible in the mid-1980s.

A transducer can be made to project a narrow beam of modulated ultrasound that is powerful enough, at 100 to 110 dBSPL, to substantially change the speed of sound in the air that it passes through. The air within the beam behaves nonlinearly and extracts the modulation signal from the ultrasound, resulting in sound that can be heard only along the path of the beam, or that appears to radiate from any surface that the beam strikes. This technology allows a beam of sound to be projected over a long distance to be heard only in a small well-defined area; a listener outside the beam hears nothing. This effect cannot be achieved with conventional loudspeakers, because sound at audible frequencies cannot be focused into such a narrow beam.

There are some limitations with this approach. Anything that interrupts the beam will prevent the ultrasound from propagating, like interrupting a spotlight’s beam. For this reason, most systems are mounted overhead, like lighting.

Applications:

Military:

There has been speculation about military sonic weapons that emit highly-directional high-intensity sound; however, these devices do not use ultrasound, although sometimes thought to do so. Wikileaks has published technical specifications of such sound weapons.

Commercial Advertising:

A sound signal can be aimed so that only a particular passer-by, or somebody very close, can hear it. In commercial applications, it can target sound to a single person without the peripheral sound and related noise of a loudspeaker.

Personal Audio:

It can be used for personal audio, either to have sounds audible to only one person, or that which a group wants to listen to. The navigation instructions for example are only interesting for the driver in a car, not for the passengers. Another possibility are future applications for true stereo sound, where one ear does not hear what the other is hearing.

History:

This technology was originally developed by the US Navy and Soviet Navy for underwater sonar in the mid-1960s, and was briefly investigated by Japanese researchers in the early 1980s, but these efforts were abandoned due to extremely poor sound quality (high distortion) and substantial system cost. These problems went unsolved until a paper published by Dr. F. Joseph Pompei of the Massachusetts Institute of Technology in 1998 fully described a working device that reduced audible distortion essentially to that of a traditional loudspeaker.

Products:

As of 2014 there were known to be five devices which have been marketed that use ultrasound to create an audible beam of sound.
Audio Spotlight:

F. Joseph Pompei of MIT developed technology he calls the “Audio Spotlight”, and made it commercially available in 2000 by his company Holosonics, which according to their website claims to have sold “thousands” of their “Audio Spotlight” systems. Disney was amongst the first major corporations to adopt it for use at the Epcot Center, and many other application examples are shown on the Holosonics website.

Audio Spotlight is a narrow beam of sound that can be controlled by the same precision as light. It uses a beam of ultrasound as a “virtual acoustic source”, enabling unprecedented control of sound distribution. The ultrasound has wavelengths only a few millimeters long which are much smaller than the source, and therefore naturally travel in an extremely narrow beam. Of course, the ultrasound, which contains frequencies far outside our range of hearing, is completely inaudible. but as the ultrasonic beam travels through the air, the inherent properties of the air cause the ultrasound to change shape in predictable way. This gives rise to frequency components in the audible band, which can be accurately predicted, and therefore precisely controlled.

HyperSonic Sound:

Elwood “Woody” Norris, founder and Chairman of American Technology Corporation (ATC), announced he had successfully created a device which achieved ultrasound transmission of sound in 1996. This device used piezoelectric transducers to send two ultrasonic waves of differing frequencies toward a point, giving the illusion that the audible sound from their interference pattern was originating at that point. ATC named and trademarked their device as “HyperSonic Sound” (HSS). In December 1997, HSS was one of the items in the Best of What’s New issue of Popular Science. In December 2002,Popular Science named HyperSonic Sound the best invention of 2002. Norris received the 2005 Lemelson-MIT Prize for his invention of a “hypersonic sound”. ATC (now named LRAD Corporation) spun off the technology to Parametric Sound Corporation in September 2010 to focus on their Long Range Acoustic Device products (LRAD), according to their quarterly reports, press releases and executive statements.Mitsubishi Electric Engineering Corporation:

Mitsubishi apparently offers a sound from ultrasound product named the “MSP-50E” but commercial availability has not been confirmed.AudioBeam:

German audio company Sennheiser Electronic once listed their “AudioBeam” product for about $4,500. There is no indication that the product has been used in any public applications. The product has since been discontinued.Soundlazer:

Started as a Kickstarter project in 2012, Richard Haberkern developed the Soundlazer SL-01 for use by the general public. The SL-01 is currently available for sale on the company’s website as the consumer version or as a developer’s kit. A new model, named the Soundlazer Snap, is expected to deliver in early 2015, following a crowdfunding campaign on Kickstarter. The Snap was designed primarily for hobbyists and requires some assembly.
For the nonlinear effect to occur, relatively high intensity ultrasonics are required. The SPL involved was typically greater than 100 dB of ultrasound at a nominal distance of 1m from the face of the ultrasonic transducer. Exposure to more intense ultrasound over 140 dB near the audible range (20–40 kHz) can lead to a syndrome involving manifestations of nausea, headache,tinnitus, pain, dizziness and fatigue, but this is around 100 times the 100 dB level cited above, and is generally not a concern. Dr Joseph Pompei of Audio Spotlight has published data showing that their product generates ultrasonic sound pressure levels around 130 dB (at 60 kHz) measured at 3 meters.

The UK’s independent Advisory Group on Non-ionising Radiation (AGNIR) produced a 180-page report on the health effects of human exposure to ultrasound and infrasound in 2010. The UK Health Protection Agency (HPA) published their report, which recommended an exposure limit for the general public to airborne ultrasound sound pressure levels (SPL) of 100 dB (at 25 kHz and above).

OSHA specifies a safe ceiling value of ultrasound as 145 dB SPL exposure at the frequency range used by commercial systems in air, as long as there is no possibility of contact with the transducer surface or coupling medium (i.e. submerged). This is several times the highest levels used by commercial Audio Spotlight systems, so there is a significant margin for safety. In a review of international acceptable exposure limits Howard et al. (2005) noted the general agreement amongst standards organizations, but expressed concern with the decision by United States of America’s Occupational Safety and Health Administration (OSHA) to increase the exposure limit by an additional 30 dB under some conditions (equivalent to a factor of 1000 in intensity.

For frequencies of ultrasound from 25 to 50 kHz, a guideline of 110 dB has been recommended by Canada, Japan, the USSR, and the International Radiation Protection Agency, and 115 dB by Sweden in the late 1970s to early 1980s, but these were primarily based on subjective effects. The more recent OSHA guidelines above are based on ACGIH (American Conference of Governmental Industrial Hygienists) research from 1987.

Lawton(2001) reviewed international guidelines for airborne ultrasound in a report published by the United Kingdom’s Health and Safety Executive, this included a discussion of the guidelines issued by the American Conference of Governmental Industrial Hygienists (ACGIH), 1988. Lawton states “This reviewer believes that the ACGIH has pushed its acceptable exposure limits to the very edge of potentially injurious exposure”. The ACGIH document also mentioned the possible need for hearing protection.

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